Method and apparatus for processing ferrous materials

ABSTRACT

In steel-making, there are conflicting processing requirements at different stages of the process. The method and apparatus of the invention creates different processing conditions at different stages of the process. Molten ferrous material (2) is stirred by bubbling a gas therethrough. A refractory ring (10) is first partially immersed in the molten steel within a substantially slag free portion of the surface of the molten ferrous metal to form a bounded substantially slag-free area. The molten steel is then heated by the introduction of exothermically reacting heating agents such as aluminium and oxygen through an oxygen gas line (7) and an aluminium delivery tube (9), respectively. The refractory ring (10) is then removed from the molten steel when the steel has reached a predetermined temperature and air is then excluded from the volume above the molten steel by placing a hood (16) over the ladle containing the molten steel. Sulphur is then removed from the steel.

This invention relates to a method of processing ferrous metalsincluding the heating of, and removal of sulphur from, a molten ferrousmetal.

The molten ferrous metal is heated by the energy produced in anexothermic reaction between heating agents. The heating agents areusually oxygen gas blown onto the surface of the ferrous metal andaluminium (or silicon) dissolved in the molten ferrous metal. Aluminium(or silicon) is added to the molten ferrous metal at a controlled rate,whilst oxygen gas is simultaneously blown onto the surface of theferrous metal at a controlled rate. However, in steel making the surfaceof the molten ferrous metal is covered with a layer of slag whichinhibits the dissolving of the aluminium (or silicon) in the ferrousmetal, and the transfer of oxygen gas to the surface of the ferrousmetal, thereby reducing the efficiency of the heating process.

Sulphur is removed from the ferrous metal once it has been deoxidised.Desulphurisation is achieved by a chemical reaction between the sulphurdissolved within the molten ferrous metal and the slag thereon and,usually, injection of lime/spar and/or calcium containing agents such ascalcium silicide. It is therefore desirable to have the maximum possibleslag/molten metal contact surface for desulphurisation. As the sulphurcontent of the molten ferrous metal is reduced, its tendency to absorbnitrogen from the atmosphere increases. The absorption of nitrogen canhave a detrimental effect on the properties of the finished product andit is therefore necessary to prevent or minimise the flow of nitrogen(or air containing nitrogen) across the surface of the molten ferrousmetal during desulphurisation so as to minimise the pick-up of nitrogenby the metal.

The present invention provides a method of processing ferrous metalswherein the molten ferrous metal is stirred by bubbling a gas throughthe molten ferrous metal. A ring is then partially immersed in themolten ferrous metal within a substantially slag free portion of thesurface of the molten ferrous metal to form a bounded substantially slagfree area. The molten ferrous metal is heated by the introduction ofexothermically reacting heating agents to the said slag free area and,the said ring is removed from the molten ferrous metal when the ferrousmetal has reached a pre-determined temperature. Air is then excludedfrom the volume above the molten ferrous metal and sulphur removedtherefrom.

Preferably the exothermically reacting agents are oxygen gas andgranulated aluminium or ferro-silicon or a mixture thereof

Preferably the method of the invention is such that air is excluded fromthe volume above the molten ferrous metal by placing a cover thereoverand maintaining the volume at a higher pressure than that of thesurrounding atmosphere.

The present invention also provides apparatus for processing ferrousmetals including a receptacle for receiving the molten ferrous metal, agas supply for bubbling gas through the molten ferrous metal, a moveablering for defining a bounded substantially slag free area on the surfaceof the molten ferrous metal, and a moveable receptacle cover forexcluding air from the volume above the surface of the molten ferrousmetal; wherein the bubbling gas produces a substantially slag freeportion on the surface of the molten ferrous metal, the said ring ismoveable between a first position such that it is partially immersed inthe molten ferrous metal substantially within the slag free portionproduced by the bubbling gas and a second position such that it isseparated from the molten ferrous metal, the receptacle cover is movablebetween a first position such that it excludes the surroundingatmosphere from the volume above the molten ferrous metal and a secondposition such that the volume is open to the surrounding atmosphere, andthe receptacle cover and said ring are moveable independently of eachother.

Preferably the apparatus of the present invention further includescontrol means for controlling the rate of supply of heating agents andmonitoring means for determining the temperature of the molten ferrousmetal wherein the control means form a closed loop control system.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of example,with reference to the accompanying drawings

IN THE DRAWINGS

FIG. 1 is a transverse section through an apparatus for steel makingembodying the present invention, with the receptacle cover in the firstposition and the ring in its second position;

FIG. 2 is a transverse section through an apparatus for steel makingembodying the present invention with the receptacle cover in its firstposition and the ring in its first position; and

FIG. 3 is a transverse section through an apparatus for steel makingembodying the present invention, with the receptacle cover in the secondposition and the ring in its first second position.

FIG. 4 is an illustration of Example 1 (Table 1);

FIG. 5 is an illustration of Example 2 (Table 2); and

FIG. 6 is an illustration of Example 3 (Table 3).

Referring to FIG. 1, a ladle 1 carried on a ladle transport car 3 andcontaining deoxidised steel 2 is enclosed within a substantiallyairtight chamber 4.

The ladle 1 is provided with an argon stir bottom plug 5. The argon stirbottom plug 5 is a porous plug through which argon gas may be introducedinto the molten steel. The steel is stirred by the introduction of theargon gas through the bottom plug 5. The vigorous bubbling of the inertargon gas stirs the ladle contents thereby homogenising the temperatureand chemical composition of the molten steel. The flow rate of the argongas can be adjusted to suit the metallurgical requirement. For example,the argon flow rate is reduced during heating to slow down the rate ofdispersion of aluminium.

The chemical composition of the steel is adjusted by the addition ofweighed amounts of deoxidants and ferro-alloys. Cooling of the liquidsteel is done by the controlled addition of cooling scrap, added in thesame way as other ferro-alloys.

There are two distinct layers in the ladle, a layer of molten steel 2and a layer of slag 14 on the steel's surface.

The bubbling argon gas produces a slag free zone on the surface of themolten steel. The size of this slag free zone varies with the argon flowrate. The boundary of the slag free zone changes with the action of thegas and the slag free zone repeatedly expands and contracts.

The chamber 4 is provided with an argon stir top lance 6, an oxygen gaslance 7, an aluminium delivery tube 9, an refractory ring 10 and a watercooled ladle hood 16.

The oxygen lance 7 has a drive 8 which controls the separation betweenthe oxygen outlet of the lance 7 and the surface of the molten steel.The position of the lance is automatically adjusted after each treatmentto compensate or the wear of the lance.

The argon stir top lance 6 may be moved between a first position suchthat the outlet end of the lance 6 is immersed in the molten steel andthe argon gas bubbles therethrough and a second position remote from themolten steel. The bubbling of the argon gas stirs the steel in a mannersimilar to that described above for the argon stir bottom plug 5. Theargon stir top lance 6 may also be used to inject powdered additivesinto the liquid steel. Such additives are injected together with argongas acting as their carrier.

A slag free zone is created on the surface of the steel by the argonbubbles breaking therethrough. This applies irrespective of whether theargon gas is introduced via the bottom plug 5 or top lance 6. Therefractory ring 10 is lowered into the substantially slag free zone whenthis is at or near its greatest extent (or when it is at least as greatas the diameter of the refractory ring). The partially immersedrefractory ring then encloses a bounded substantially slag free area.This area will then remain substantially slag free whilst the ring 10 isso immersed. The ring 10 is a refractory faced steel cylinder supportedby three arms 20, raised and lowered by a three rope winch systemoperated by the ring winch 18. The ring may have any circumferentialshape provided that it defines a central aperture for enclosing the slagfree area i.e. the ring need not be circular. When the ring has beenlowered into its treatment position, the three vertical hangers, 35which support the ring are clamped by three clamps, 36 to preventfurther movement of the ring.

It is important to note that the use of the refractory ring has abeneficial effect on the efficiency of heating. The volume of metalenclosed within the ring is relatively small, at any given time,although the liquid steel within the ring is changed and recirculated bythe effect of the argon stirring. Adding the aluminium to a relativelysmall volume of liquid steel means that oxygen is blown at a locallyaluminium rich liquid. This encourages the reaction between oxygen andaluminium which is the required exothermic reaction rather than theundesired reaction between oxygen and other oxidisable elements such asiron, manganese and carbon. The use of the refractory ring thereforereduces losses of carbon and manganese quite significantly, as opposedto heating methods where the aluminium addition is rapidly diluted, aswould be the case for example if the aluminium was added to the fullvolume of the steel in the ladle.

To increase the efficiency still further, the rate of aluminium additionis automatically controlled and monitored throughout the heatingprocess, depending on the required rate of heating. The oxygen flow rateis also controlled and monitored throughout the heating process, instoichiometric ratio with the addition rate of the aluminium, avoidingthe oxidation of other elements. The ability of the apparatus of theinvention to enclose a relatively small, slag free volume of steel istherefore highly advantageous.

A camera 26 is provided which allows the visual monitoring of thesurface of the steel and/or slag within the ladle 1.

The argon top lance 6 may be lowered and immersed in the molten steel soas to promote stirring thereof as described above. The argon lance 6 mayalso be used for injecting suitable powdered desulphurisation agents(e.g. lime/spar and/or calcium containing agents such as calciumsilicide).

It should be noted that although the bulk of the desulphurisation isdone by the injection of a suitable powdered desulphurisation agentthrough an immersed top lance, significant desulphurisation results fromcontact between the slag layer and liquid steel layer. Maximumslag/molten steel surface contact is required so as to renderdesulphurisation as efficient as possible. For desulphurisation,therefore, the full surface area of the molten steel should be incontact with the slag. In addition, to maintain the highly reducingconditions required for desulphurisation and so as to minimise take upof nitrogen by the molten steel, the ingress of atmospheric oxygen andnitrogen respectively into the ladle must be minimised. This means thatalthough the full area of the surface of the molten steel in the ladleshould be exposed to the top slag layer, air should be excluded from thevolume 13 above the molten steel and the slag.

The ladle hood 16 is water cooled and consists of two distinct portions,a moveable ladle cover portion 27 and a fixed ladle cover portion 28with a seal member 32 therebetween. The moveable cover portion 27 may bemoved from a first position where it is resting on the ladle trunnionband reinforcing flange 33 to a second position separated from flange 33so as to allow communication between the volume 13 and the surroundingatmosphere. A positive pressure (larger than that of the surroundingatmosphere in the chamber 4) is created within the ladle cover 16 whenthis is in its first position. The ladle hood is sealed on the ladletrunnion band reinforcing flange, 33 rather than the ladle lip since themating surface is much cleaner and flatter, thereby providing a moreeffective seal.

The moveable ladle cover portion 27 is raised and lowered by a hoodwinch 17. The ladle hood winch 17 and the refractory ring winch 18 areindependently operable. This allows one to adjust the conditions withinthe ladle so as to maximise the efficiency of the heating anddesulphurisation steps. This feature of the present invention allows oneto maximise the efficiency of the steel making process despite theconflicting metallurgical requirements for the heating anddesulphurisation processes.

The aluminium delivery tube 9 is fed from a hopper 34 via a vibratoryfeeder 29 and an addition hopper system 30. This addition hopper systemprevents the emission of fumes during alloying, or the drawing of airinto the volume 13 above the surface of the steel in the ladle 1. Theairtight chamber 4 within which the ladle and its associated equipmentare enclosed is provided with at least one fume extraction duct 31. Thisensures that fume extraction is carried out from the chamber 4surrounding the ladle, ladle transport car 3 and ladle hood 16 ratherthan directly from the ladle hood 16. The rear of the ladle transportcar 3 is provided with a plate which coincides with the walls of thechamber 4 so as to form an airtight enclosure when the ladle transportcar is in position within the apparatus of the present invention.

The pressure differential between the fixed hood and surroundingenclosure is constantly monitored during treatment. The measureddifferential is compared with a required figure. Any variation betweenthe required and actual value is used to open or close an air ballastvalve 37 in the exhaust duct. The air ballast valve, which is abutterfly type valve increases or decreases the air loading to the fumeextraction system, effectively increasing or decreasing the extractioncapacity. Modulating the extraction capacity is required to ensure apositive pressure within the fixed hood relative to the surroundingenclosure.

The method and apparatus described above allow one to optimise theconditions for distinct metallurgical operations having sometimesconflicting requirements.

The process is now illustrated by means of the following examples oftime - temperature curves.

Example 1 As shown in FIG. 4 (Table 1)

The steel arrives at the installation in the unkilled (oxidised) state.

Step 1 The steel is argon stirred using bottom mounted porous plug, ortip lance, for homogenisation purposes.

Step 2 The steel is deoxidised by aluminium addition via the double gateaddition hopper and alloy system, whilst the steel is argon stirred.

Step 3 The steel is sampled, and a temperature taken.

Step 4 The steel is reheated by oxygen top blowing with the refractoryring immersed by the simultaneous addition of aluminium (orferro-silicon) whilst argon stirring using bottom plug or top lance.

Step 5 Based on the steel analysis, ferro-alloy additions are made viathe double gate addition hopper and alloy system whilst argon stirringusing bottom plug or top lance.

Step 6 The steel is desulphurised by the injection of lime/spar powderthrough a top lance using argon as a carrier gas.

Step 7 Sulphide modification is done by the injection of calcium intothe steel in the form of a cored wire via a wire feed machine.

Step 8 The steel is sampled, and a temperature taken.

Step 9 The steel is reheated by oxygen top blowing with the simultaneousaddition of aluminium (or ferro-silicon) whilst argon stirring usingbottom plug or top lance.

Step 10 The steel is sampled, and a temperature taken.

Example 2 as shown in FIG. 5 (Table 2)

The steel arrives at the installation in the unkilled (oxidised) state.

Step 1 The steel is argon stirred using a bottom mounted porous plug ortop lance, for homogenisation purposes.

Step 2 The steel is deoxidised by aluminium addition.

Step 3 The steel is sampled, and a temperature taken.

Step 4 Based on the steel analysis, ferro-alloy additions are made viathe double gate addition hopper and alloy system whilst argon stirringusing bottom plug or to lance.

Step 5 Alloying of special elements or for accurate analysis control canbe carried out using cored or solid wires via the wire feed machinewhilst argon stirring using bottom plug or top lance.

Step 6 The steel is reheated by oxygen top blowing with the refractoryring immersed by the simultaneous addition or aluminium (orferro-silicon) whilst argon stirring using bottom plug or top lance.

Step 7 The steel is desulphurised by the injection of lime/spar powderthrough a top lance using argon as a carrier gas.

Step 8 The steel is sampled, and a temperature taken.

Step 9 The steel is reheated by oxygen top blowing with the simultaneousaddition of aluminium (or ferro-silicon) whilst argon stirring usingbottom plug or top lance.

Step 10 Final fine trimming additions are made based on the steelanalysis using ferro-alloys via the double gate addition hopper andalloy system whilst argon stirring using bottom plug or top lances.

Step 11 The steel is sampled, and a temperature taken.

Example 3 as shown in FIG. 6 (Table 3)

Step 1 The steel is argon stirred using a bottom mounted porous plug ortop lance, for homogenisation purposes.

Step 2 The steel is deoxidised by aluminium addition.

Step 3 The steel is sampled, and a temperature taken.

Step 4 Based on the steel analysis, cored or solid wire additions aremade via the wire feed machine whilst argon stirring using the bottomplug or top lance.

Step 5 The steel is reheated by oxygen top blowing with the simultaneousaddition of aluminium (or ferro-silicon) whilst argon stirring usingbottom plug or top lance.

Step 6 Based on the steel analysis, ferro-alloy additions are made viathe double gate addition hopper and alloy system whilst argon stirringusing bottom plug or top lance.

Step 7 The steel is desulphurised by the injection of lime/spar powderthrough a top lance using argon as a carrier gas.

Step 8 Sulphide modification is done by the injection of calcium intothe steel in the form of a cored wire via a wire feed machine.

Step 9 The steel is sampled, and a temperature taken.

Step 10 Final fine trimming additions are made based on the steelanalysis using ferro-alloys via the double gate addition hopper andalloy system whilst argon stirring using bottom plug or top lances.

Step 11 The steel is reheated by oxygen top blowing with thesimultaneous addition of aluminium (or ferro-silicon) whilst argonstirring using bottom plug or top lance.

Step 12 The steel is sampled, and a temperature taken.

I claim:
 1. A method of processing molten ferrous metals containing atleast some sulphur in a metallurgical vessel having a metal melttherein, said melt having an upper surface, said upper surface havingslag thereon and said metallurgical vessel including a movable ring, themethod including the steps of:producing a substantially slag freesurface portion of the surface of said melt; partially immersing saidring in said melt in said substantially slag-free surface portion toform a bounded substantially slag-free area of said surface; deliveringexothermically reacting heating agents to the said slag-free area toheat said melt; removing said ring from said melt when said melt hasreached a selected temperature; enclosing and sealing a volume above andbounded by said melt so as to exclude air from said volume above themelt; and removing sulphur from said melt.
 2. The method of claim 1 inwhich said exothermically reacting agents are oxygen gas and granulatedmaterial selected from the group consisting of aluminum, ferro-siliconor a combination thereof.
 3. The method of claim 1 in which said step ofremoving sulphur comprises the step of introducing of powdered materialselected from the group consisting of lime, spar, calcium silicide or acombination thereof, into said melt.
 4. The method of claim 2 in whichsaid step of removing sulphur comprises the introduction of powderedmaterial selected from the group consisting of lime, spar, calciumsilicide or a combination thereof, into said melt.
 5. The method ofclaim 1 in which said step of excluding air from said volume above saidmelt comprises the step of placing a cover over said volume andmaintaining said volume at a higher pressure than that of thesurrounding atmosphere.
 6. The method of claim 2 in which said step ofexcluding air from said volume above the melt comprises the step ofplacing a cover over said volume and maintaining said volume at a higherpressure than that of the surrounding atmosphere.
 7. The method of claim3 in which said step of excluding air from said volume above the meltcomprises the step of placing a cover over said volume and maintainingsaid volume at a higher pressure than that of the surroundingatmosphere.
 8. Apparatus for processing ferrous metals comprising; areceptacle for receiving molten ferrous metal, a gas supply for bubblinggas through said molten ferrous metal in said receptacle, a ring movablewithin and relative to said receptacle between a position partiallywithin said molten ferrous metal and a position out of said moltenmetal, a receptacle cover movable relative to said receptacle to openand close said receptacle, a first control which controllably moves saidring, and a second control which controllable moves said receptaclecover, said first and second controls being independently operable sothat said receptacle cover and said ring are movable independently ofeach other.
 9. Apparatus according to claim 8 wherein said ring enclosesan area within said receptacle and the apparatus further comprises aheating agent supply which supplies exothermically reacting heatingagents to said area enclosed by said ring within said receptacle. 10.Apparatus according to claim 9 wherein said heating agent supply sppliesoxygen gas and granulated material selected from the group consisting ofaluminum, ferrosilicon or a combination thereof.
 11. Apparatus accordingto claim 8 further comprising means for supplying powdered materialsselected from the group consisting of lime, spar, calcium silicide or acombination thereof, to said receptacle whilst said receptacle cover isclosing said receptacle.
 12. Apparatus according to claim 9 furthercomprising means for supplying powdered materials selected from thegroup consisting of lime, spar, calcium silicide or a combinationthereof to said receptacle whilst said receptacle cover is closing saidreceptacle.
 13. Apparatus according to claim 10 further comprising meansfor supplying powdered materials selected from the group consisting oflime, spar, calcium silicide or a combination thereof to said receptaclewhilst said receptacle cover is closing said receptacle.
 14. Apparatusaccording to claim 10 including an addition hopper system for supplyingsaid granulated material.
 15. Apparatus according to claim 8 whereinsaid receptacle cover closes said receptacle to enclose a volume aboveand bounded by said receptacle and the contents of said receptacle, saidenclosed volume being separated by said receptacle and receptacle coverfrom a space surrounding said closed receptacle, said receptacleincluding:pressure measurement means for measuring the relativepressures of the volume enclosed within said closed receptacle and saidspace surrounding said closed receptacle; and a fume extraction lineconnected to said space and having controllable fume extraction means toextract gases or fumes from said space, said fume extraction means beingconnected to said pressure measurement means; said measurement ofrelative pressures made by said pressure measurement means controllingthe fume extraction means to maintain said enclosed volume at a higherpressure than said space surrounding said receptacle.
 16. Apparatusaccording to claim 11 wherein said receptacle cover closes saidreceptacle to enclose a volume above and bounded by said receptacle andthe contents of said receptacle, said enclosed volume being separated bysaid receptacle and receptacle cover from a space surrounding saidclosed receptacle, said receptacle including:pressure measurement meansfor measuring the relative pressures of the volume enclosed within saidclosed receptacle and said space surrounding said closed receptacle; anda fume extraction line connected to said space and having controllablefume extraction means to extract gases or fumes from said space, saidfume extraction means being connected to said pressure measurementmeans; said measurement of relative pressures made by said pressuremeasurement means controlling the fume extraction means to maintain saidenclosed volume at a higher pressure than said space surrounding saidreceptacle.
 17. Apparatus according to claim 12 wherein said receptaclecover closes said receptacle to enclose a volume above and bounded bysaid receptacle and the contents of said receptacle, said enclosedvolume being separated by said receptacle and receptacle cover from aspace surrounding the closed receptacle, said receptacleincluding:pressure measurement means for measuring the relativepressures of the volume enclosed within said closed receptacle and saidspace surrounding said closed receptacle; and a fume extraction lineconnected to said space and having controllable fume extraction means toextract gases or fumes from said space, said fume extraction means beingconnected to said pressure measurement means; said measurement ofrelative pressures made by said pressure measurement means controllingthe fume extraction means to maintain said enclosed volume at a higherpressure than said space surrounding said receptacle.
 18. Apparatusaccording to claim 15 wherein said space surrounding said receptacle andreceptacle cover is enclosed within a chamber provided with a fumeextraction duct having an air ballast valve.
 19. Apparatus according toclaim 8 further comprising a heating control which controls the rate ofsupply of said exothermically reacting heating agents to saidreceptacle, and a monitor for determining the temperature of thecontents of said receptacle, said monitor producing a temperature signalrepresentative of the temperature of the contents of said receptacle,said heating control including a comparator for comparing said monitortemperature signal to a signal representative of a desired temperatureto produce a comparison signal, said rate of supply of said heatingagents being controllably varied as a function of said comparisonsignal.
 20. Apparatus according to claim 8 wherein said receptacle andreceptacle cover are enclosed within a chamber provided with a fumeextraction duct.